1. Field of Invention
This invention relates to stencil printing machines, and more particularly to a stencil printing machine which performs necessary operations depending on whether or not a stencil sheet is wound on the rotary cylindrical drum.
2. Description of Related Art
A stencil printing machine is known in the art in which a stencil sheet is supplied to the rotary cylindrical drum set at a predetermined position at which a stencil sheet is to be wound (hereinafter referred to as "a home position", when applicable), and then it is wound on the drum by turning the drum. The machine has been disclosed, for instance, by Examined Japanese Patent Publication No. Sho. 62-30117.
With a stencil printing machine of this type, in order to wind a stencil sheet (hereinafter referred to as "a new stencil sheet", when applicable) on the rotary cylindrical drum, the following operations are carried out: First, it is determined whether or not another stencil sheet (hereinafter referred to as "an old stencil sheet", when applicable) has been wound on the drum. When the old stencil sheet is on the drum, a stencil discharging process is performed to remove the old stencil sheet from the drum. After the stencil discharging process, a stencil winding process is carried out to wind the new stencil sheet on the drum. On the other hand, in the case where it is determined that no stencil sheet is on the drum, the stencil winding process is started immediately.
After it is ensured that the new stencil sheet has been wound on the drum, a sheet supplying operation is started; that is, a stencil printing operation is started.
Conventionally, in order to determine whether or not a stencil sheet has been wound on the rotary cylindrical drum, a stencil presence/absence detection is carried out as follows: A sensor reaction piece of non-reflection type made of black sponge or the like is provided in the ink non-passage part of the tubular wall of the drum. The sensor reaction piece is located near a stencil clamp and is covered with a stencil sheet, while a reflection type sensor is fixedly mounted on the machine frame in such a manner as to confront the tubular wall of the drum, so that the quantity of light received by the sensor is changed depending on whether or not the sensor reaction piece is covered with a stencil sheet. That is, it is determined from the variations in the quantity of light received by the sensor whether or not a stencil sheet is wound on the drum. In this stencil presence/absence detection, it is necessary to turn the drum until the sensor reaction piece on the drum comes to a predetermined position where it confronts the sensor (hereinafter referred to as "a stencil presence/absence detecting position", when applicable).
If the stencil presence/absence detecting position is the position of rotation where a stencil winding operation should be started (hereinafter referred to as "a stencil-winding-operation starting rotational position", when applicable), then it is unnecessary to turn the drum thereby to determine whether or not the old stencil sheet is left on the drum. However, in this case, it cannot be detected immediately after the start of the stencil winding operation whether or not the new stencil sheet is correctly wound on the drum (being satisfactorily clamped).
Hence, it is necessary to provide two sensors, one sensor whose stencil presence/absence detecting position is the stencil-winding-operation starting rotational position, and the other sensor whose stencil presence/absence detecting position is the position which the drum takes being turned through a small angle, for instance 70.degree. to 80.degree., from the stencil-winding-operation starting rotational position.
For a detailed description of a stencil presence/absence detecting operation using those two sensors, it is requested to refer to the specification and the drawings of Unexamined Japanese Patent Publication No. Hei. 7-25127.
On the other hand, in the case where only the sensor whose stencil presence/absence detecting position is the position which the drum takes after being turned through a small angle is employed, it is unnecessary to provide two sensors; however, before the new stencil sheet is wound on the drum, it is necessary to cause the drum to make one rotation thereby to determine whether or not the old stencil sheet is on the drum. Furthermore, in order to detect whether or not the used stencil sheet is left on the drum before the start of the printing operation, the drum must be turned one rotation. This increases the time required for the stencil winding operation and accordingly for the stencil printing operation; more specifically, the time which elapses from the time instant that the stencil-making-operation start key or the stencil-printing-operation start key is operated until the first stencil print is obtained.